Following explanation from the mitral position because of primary tissue failure, 30 human antibiotic sterilised stent mounted aortic valve allografts and 28 glutaraldehyde treated porcine xenografts were examined for evidence of tissue detachment from the stents. These grafts had been in situ for 34 to 166 months. Graft detachment had occurred from one or two stent posts with displacement of the commissures and central valvular incompetence in 12 (67%) of 18 allografts supported on rigid stainless steel stents, in six (50%) of 12 allografts mounted on flexible acetal copolymer stents, but in only one (4%) of 28 xenografts mounted on polypropylene stents. In regions of detachment the aortic remnant of the graft was infiltrated by components of blood and phagocytic cells which had removed not only fibrin but also graft tissue. Detachment did not occur from stent posts where the graft margin had become coated by collagenous host tissue. This intimal fibrous sheath appeared not only to strengthen the attachment of the graft but also to limit the entry of fibrin and phagocytes into the graft tissue.